1. Field of the Invention
This invention relates to a bracket that is used in orthodontic therapy and has an archwire slot liner for enhancing sliding movement of the bracket along an archwire coupled to the bracket.
2. Description of the Related Art
In orthodontic treatment, tiny devices known as brackets are secured to the patient's teeth. An archwire is received in a slot of each bracket, and is held in place in the slots by ligating wires or by small elastic 0-rings that extend around each bracket and the archwire. The teeth connected to the brackets are urged toward orthodontically correct positions by bends or twists placed in the archwire, or by elastomeric modules interconnecting certain brackets. The archwire serves as a track to guide sliding movement of the brackets so that the associated teeth are shifted toward desired positions.
In the past, orthodontic brackets were often made of stainless steel, and archwires were made of stainless steel or alloys containing stainless steel, nickel and titanium. In general, frictional resistance to sliding movement of the metal brackets, while not insignificant, is a factor that is not considered unsatisfactory by most orthodontists. However, metal brackets are not aesthetic and are sometimes referred to as a "tin grin" that may be an embarrassment to the patient.
Orthodontic brackets made of non-opaque plastic materials such as polycarbonate have been introduced by various manufacturers over the years. Unfortunately, some plastic brackets exhibit undue deformation of the archwire slots because of creep of the material as orthodontic forces are applied by the wire to the brackets. Undue deformation of the archwire slots may prevent precise control of movement of the associated teeth, and in some instances may cause the brackets to fracture. Replacement of brackets during orthodontic treatment is time consuming and is often considered a nuisance by the orthodontist as well as by the patient.
It has been proposed in the past to provide metallic archwire slot liners for plastic brackets, in part as an attempt to avoid deformation of the plastic material. Examples of archwire slot liners are described in U.S. Pat. Nos. 3,964,165, 4,299,569 and 4,302,532. Metallic archwire slot liners for plastic brackets provide sliding mechanics that resemble the sliding mechanics as would be observed when an all-metal bracket is used.
Orthodontic brackets have also been made of translucent ceramic material such as polycrystalline aluminum oxide as is described in U.S. Pat. No. 4,954,080, assigned to the assignee of the present invention. Ceramic is a relatively hard material in comparison to plastic and does not exhibit creep deformation in areas adjacent the archwire slot when subjected to forces of the archwire. However, application of an undue force by the archwire may fracture the bracket, possibly because of localized areas of relatively high stress concentrations.
It has also been observed that the sliding mechanics of a metallic archwire in the slot of a ceramic bracket are not as satisfactory as the sliding mechanics of a metallic wire in the slot of a metal bracket. In one experiment, scanning electron microscope photographs of a stainless steel archwire showed that the archwire had deep scratches after movement along the slot of a ceramic bracket, suggesting that the ceramic material galled the archwire and provided severe mechanical restraint to sliding movement. Unfortunately, undue resistance of a ceramic bracket to sliding movement along an archwire may lengthen treatment time and thus provide an additional expense to both the orthodontist and the patient.